How to identify bacteria growing on nutrient agar plates

Updated March 23, 2017

Evaluating bacteria on agar plates is one method of identification used by microbiologists. Colony morphology, or the physical characteristics of bacteria as it multiplies, generally cannot be seen using other diagnostic methods. The shape, colour, surface appearance, and even smell of a bacterial colony can help confirm an identification. Nutrient agar plates are used to grow the bacteria so they can be contained in a small visible area for laboratory identification. As scientists collect information, the true identity of the bacteria can be gleaned.

Don appropriate lab attire. Put on your lab coat and any other protective gear, such as goggles, gloves, or a mask, required by your lab protocol.

Look at the basic form of the bacterial colonies. Determine whether the form is circular, irregular, filamentous, rhizoid, or whatever is the case. You may even have more than one species present on a plate.

Determine what the elevation of the colony is. Turn the Petri dish on its side and shine illumination across the tops of the colonies to better see the cross sectional shape. Common forms are raised, convex, flat, umbonate, and crateriform.

See whether you can determine the margin of the colonies. Use a 40x objective lens or higher on your light microscope to see if you can tell the margin or shape around the colony's edges. Shapes are classified as entire, undulate, filiform, curled, or lobate.

Check the surface appearance. Determine whether it's smooth, glistening, rough, or dull.

Note the opacity of the colonies. Defining traits are transparent, opaque, translucent, or iridescent.

Observe any pigmentation evident on the agar plate. Some bacteria produce water-soluble pigments whereas others produce fat-soluble pigments. Put a small bit of colony into oil and shake it. If the oil colours, the bacteria is a fat-soluble variety.

See whether any additional observations can be made about your plates. Look at the consistency and check for odour.

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About the Author

Erin Moseley is an advocate for science education. Since 1985, she has written numerous technical, user and training manuals for major corporations, public agencies and universities. She holds a Bachelor of Science in geology.